This invention relates to communication systems and, more particularly, to mobile terminals operating with two or more wireless communications networks.
Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as that designated as IS-54B (and its successor IS-136) in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993. In addition, satellite based radio communication systems are also being utilized to provide wireless communications in various regions such as the Asian Cellular Satellite System (ACeS) generated by Lockheed Martin Corporation. Furthermore, dual-mode mobile terminals are known which allow a single terminal to access to different networks. For example, an analog/digital dual-mode terminal or a terrestrial/satellite dual-mode terminal may be desirable in various geographic areas to maximize the communications capabilities available to a user.
FIG. 1 illustrates a conventional terrestrial wireless communication system 20 that may implement one of the aforementioned wireless communication standards. The wireless system may include one or more wireless mobile terminals 22 that communicate with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between wireless mobile terminals 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system 20, a duplex radio communication link 30 may be effected between two wireless mobile terminals 22 or between a wireless mobile terminal 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications between the cell 24 and the wireless mobile terminal 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals.
FIG. 2 illustrates a conventional celestial wireless communication system 40. The celestial wireless communication system 40 may be employed to perform similar functions to those performed by the conventional terrestrial wireless communication system 20 of FIG. 1. In particular, the celestial wireless communication system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and satellite wireless mobile terminals 23. The satellite 42 communicates with the satellite wireless mobile terminals 23 and earth stations 44 via duplex communication links 46. Each earth station 44 may, in turn, be connected to a PSTN 34, allowing communications between the wireless mobile terminals 23 and conventional landline telephones 32 (FIG. 1).
The celestial wireless communication system 40 may utilize a single antenna beam covering the entire area served by the system, or as shown in FIG. 2, the celestial wireless communication system 40 may be designed such that it produces multiple, minimally-overlapping beams 48, each serving a distinct geographical coverage area 50 within the system""s service region. A satellite 42 and coverage area 50 serve a function similar to that of a base station 26 and cell 24, respectively, of the terrestrial wireless communication system 20.
Thus, the celestial wireless communication system 40 may be employed to perform similar functions to those performed by conventional terrestrial wireless communication systems. In particular, a celestial radiotelephone communication system 40 has particular application in areas where the population is sparsely distributed over a large geographic area or where rugged topography tends to make conventional landline telephone or terrestrial wireless infrastructure technically or economically impractical.
Traditional analog radiotelephone systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well-known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. These discrete frequency bands serve as channels over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell. In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands, a system designated EIA-553 or IS-19B. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 Mhz.
A defined range of radio channels are allocated to cellular mobile communications in the United States. The limitations on the number of available frequency bands present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system generally requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of frequency bands available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Fixed frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system of only several hundred frequency bands.
An alternative approach to fixed frequency reuse (with or without frequency hopping) is adaptive channel allocation (ACA). In ACA networks, the available channels are typically dynamically allocated throughout the network to maximize system capacity rather than defining a specific subset of the available channels for each cell within the network. The allocation may be based on measurements made by the mobile of channels (or frequencies) which are potential sources of interference signals as contrasted with the selection of candidate channels for handoff as provided with mobile assisted handoff in some fixed frequency reuse networks. These measurements are made to determine the level of interference signals on the various channels. The interference signal measurements may, in turn, be used to select a channel which may provide, for example, acceptable performance at the lowest transmission power level. Examples of such systems are further described in U.S. Pat. No. 5,491,837, which is incorporated herein by reference in its entirety.
U.S. Pat. No. 5,839,075 to Haartsen and Dent, which is incorporated herein by reference, describes yet another method of using measurements made by the mobile phone to assist channel allocation. The system described therein concerns a so-called xe2x80x9chome base stationxe2x80x9d which allows a cellular phone to be used as a domestic cordless phone when at home or as a mobile phone when not at home. The problem discussed and solved therein relates to obtaining a cellular frequency for the home base station""s use that is not a cellular frequency used by the cellular system in the area in which the home base station is installed. The cellular phone may be parked in the home base station xe2x80x9ccradlexe2x80x9d when not in use, which also serves to charge its battery. The physical connection of the two units allows the home base station to control the cellular phone receiver to search the cellular channels and then picks a channel with a low signal strength, indicating that it may not be used by the cellular system close by. The problem solved by the method of this patent is not the same as the problem addressed in the current application, as the home base station is not part of a network using ACA but a single, independent station. Moreover, reports of mobile measurements cannot be conveyed using the normal mobile assisted handover (MAHO) mechanism as this would typically have required radio contact already to have been established between the home base station and the phone. Thus, the problem solved in this patent is not the same as ACA for allocating a base station and a channel of a cellular network to serve a particular call, based on measurements which the cellular phone is allowed to transmit by radio to the cellular network.
Another technique which may further increase channel capacity and spectral efficiency is time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Although communication on frequency bands typically occur on a common TDMA frame that includes a plurality of time slots, communications on each frequency band may occur according to a unique TDMA frame, with time slots unique to that band. Examples of systems employing TDMA are the dual analog/digital IS-54B standard employed in the United States, in which each of the original frequency bands of EIA-553 is subdivided into 3 time slots, and the European GSM standard, which divides each of its frequency bands into 8 time slots (or burst periods) which define a frame. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user""s assigned time slots.
A channel in a TDMA system typically includes one or more time slots on one or more frequency bands. As discussed above, traffic channels are used to communicate voice, data or other information between users, for example, between a mobile terminal such as a radiotelephone and a landline telephone. In this manner, each traffic channel forms one direction of the duplex communications link established by the system from one user to another. Traffic channels typically are dynamically assigned by the system when and where needed. In addition, systems such as the European GSM system, may xe2x80x9cfrequency hopxe2x80x9d traffic channels, i.e., randomly switch the frequency band on which a particular traffic channel is transmitted. Frequency hopping reduces the probability of interference events between channels, using interferer diversity and averaging to increase overall communications quality. However, while frequency hopping might allow denser reuse of control channel frequency bands, it often is not employed because an unsynchronized radiotelephone generally would have difficulty capturing a frequency-hopped control channel due to lack of a reference point for the frequency hopping sequence employed.
Typically included in the dedicated control channels transmitted in a cell are forward control channels which are used to broadcast control information in a cell of the wide area cellular network to radiotelephones which may seek to access the network. The control information broadcast on a forward control channel may include such things as the cell""s identification, an associated network identification, system timing information and other information needed to access the wide area cellular network from a radiotelephone. Forward control channels, such as the Broadcast Control Channel (BCCH) of the GSM standard, typically are transmitted on a dedicated frequency band in each cell. A radiotelephone seeking access to a system generally xe2x80x9clistensxe2x80x9d to a control channel in standby mode, and is unsynchronized to a base station or satellite until it captures a base station or satellite control channel.
Assignment of a mobile terminal to a communication channel in a network and handoff (or handover) of a registered mobile terminal between base stations of networks employing frequency reuse (with or without frequency hopping) as well as those using ACA may be based, in part, on measurements made by the mobile terminal. A mobile terminal (cellular phone) in such a network typically includes a radio receiver including an antenna for receiving signals and an amplifier/detector for generating a measure of the strength of received signals or noise. A signal strength measure, commonly known as Radio Signal Strength Indication (RSSI), may be expressed as a logarithmic measure of received signal strength (as discussed in U.S. Pat. No. 5,048,059, which is hereby incorporated by referenced herein in its entirety) and may be converted to a digital form by an analog to digital converter.
It is known in the prior art that radio signal strength measurements can be useful in determining which base station should serve a cellular phone and/or which channel should be used for communications during a call in both fixed frequency reuse and ACA networks. For example, cellular phones using a TDMA method conforming to either the European cellular standard known as GSM or either of the American TDMA standards known respectively as D-AMPS or PCS1900 may use spare time between transmit and receive timeslots to change frequency and monitor the signal strengths of other base stations. Several measurements of signal strength are typically averaged for the same base station. The averages are reported to the currently serving base station, which determines if a handoff should be made to another, stronger base station. In contrast, an adaptive channel allocation GSM system may obtain interference signal measurements from the mobile terminal for use in channel allocation.
Dual mode mobile terminals have been proposed which are able to operate with two (or more) different types of networks. Examples of various such devices using dual circuitry in a single handset are provided in U.S. Pat. No. 5,668,837 (GSM/AMPS) and U.S. Pat. No. 5,663,957 (terrestrial/cellestial). An alternative approach for providing service from a public and a private network is discussed in U.S. Pat. No. 5,428,668. Nonetheless, a problem with existing single mode mobile terminals is that they are generally used only in a particular type of network. Accordingly, as alternative providers enter a service area or a user moves to different geographic regions where different types of networks are in use, the ability to support mobile assisted operations may be limited.
It is, therefore, an object of the present invention to provide a communications system which may allow a single mode mobile terminal to operate in different types of communication networks.
In order to provide for the foregoing and other objectives, a method is provided allowing a single mode mobile terminal to support mobile assisted signal strength measurement operations in both a fixed frequency reuse based communication network and an adaptive channel allocation based communication network. Candidate base station signal strength measurements are requested by a fixed frequency reuse type network, measured by the mobile terminal and provided to the fixed frequency reuse type network which is seeking to identify a strongest signal for mobile assisted handover operations. In addition, interference signal strength measurements are requested by an adaptive channel allocation type network, measured by the mobile terminal and provided to the adaptive channel allocation type network by the mobile terminal. No redundant circuitry is required in the mobile terminal. Instead, the mobile terminal executes the same operations using the same hardware regardless of whether the requested measurement is of a candidate signal strength or an interference signal.
In one embodiment of the present invention, a method is provided for communicating between a mobile terminal and both a first wireless communication network and a second wireless communication network. The mobile terminal receives over a control channel of the first wireless communication network an identification of a candidate channel for handoff and determines an associated signal strength of the candidate channel. The determined associated signal strength of the candidate channel is transmitted by the mobile terminal to the first wireless communication network. The mobile terminal further receives over a control channel of the second wireless communication network an identification of a plurality of interference channels and determines associated signal strengths for the plurality of interference channels. The determined associated signal strengths of the plurality of interference channels are transmitted by the mobile terminal to the second wireless communication network.
In a further embodiment of the present invention, an identification of a plurality of candidate channels corresponding to channels used by selected base stations of the first wireless communication system is received by the mobile terminal and associated signal strengths of the plurality of candidate channels are determined by the mobile terminal. The associated signal strengths of the plurality of candidate channels are then transmitted to the first communication network. The selected base stations may be base stations in the vicinity of a base station of the first wireless communication network which transmitted the identification of a plurality of candidate channels received by the mobile terminal. The first wireless communication network in one embodiment is a GSM network using at least one of fixed frequency re-use or frequency hopping.
In another embodiment of the present invention, an identification of a plurality of interference channels corresponding to channels allocated to the second wireless communication network which are currently unused by selected base stations of the second wireless communication network is received by the mobile terminal. The selected base stations of the second wireless communication network may be base stations in the vicinity of a base station of the second wireless communication network which transmitted the identification of a plurality of interference channels received by the mobile terminal. The second wireless communication network may be a GSM network using adaptive channel allocation and the first wireless communication network may be a GSM network using at least one of fixed frequency re-use or frequency hopping.
In a further embodiment of the present invention, the receiving operations of the mobile terminal are carried out using a common receiver of the mobile terminal and the determining steps are carried out using a common signal strength measurement circuit of the mobile terminal and the transmitting steps are carried out using a common transmitter of the mobile terminal.
In a further aspect of the present invention, the first wireless communication network transmits the identification of a plurality of candidate channels and receives the transmitted associated signal strengths. The first wireless communication network then selects one of the selected base stations of the first wireless communication network to provide communication support for the mobile terminal based on the received associated signal strengths. One of the selected base stations of the first wireless communication network having the strongest associated signal strength is preferably selected to provide communications service for the mobile terminal. In addition, the second wireless communication network may transmit the identification of a plurality of interference channels and receive the transmitted associated signal strengths of the plurality of interference channels. The second wireless communication network may then select a channel of the second wireless communication network to provide communications service for the mobile terminal based on the received associated signal strengths of the plurality of interference channels.
In a system aspect of the present invention, a communication system is provided including a first and second wireless communication network and a mobile terminal. The first wireless communication network includes a plurality of base stations and a means for transmitting an identification of a plurality of candidate channels associated with the plurality of base stations. The first wireless communication network further includes a means for receiving associated signal strengths of the plurality of candidate channels from a mobile terminal and a means for selecting one of the plurality of base stations of the first wireless communication network to provide communication support for the mobile terminal based on the received associated signal strengths of the plurality of candidate channels. The second wireless communication network includes a means for transmitting an identification of a plurality of interference channels, a means for receiving associated signal strengths of the plurality of interference channels from the mobile terminal and a means for selecting a channel of the second wireless communication network to provide communication support for the mobile terminal based on the received associated signal strengths of the plurality of interference channels.
The mobile terminal includes a means for receiving over a control channel of the first wireless communication network the identification of a plurality of candidate channels associated with the plurality of base stations and for determining the associated signal strengths of the plurality of candidate channels. The mobile terminal further includes a means for transmitting the determined associated signal strengths of the plurality of candidate channels to the first wireless communication network. The mobile terminal also includes a means for receiving over a control channel of the second wireless communication network the identification of a plurality of interference channels and for determining the associated signal strengths of the plurality of interference channels and transmitting the associated signal strengths of the plurality of interference channels to the second wireless communication network.
The second wireless communication network may be a GSM network using adaptive channel allocation and the first wireless communication network may be a GSM network using at least one of fixed frequency re-use or frequency hopping.
Accordingly, the present invention provides systems and methods which may allow operation of a single mode terminal supporting different types of mobile assisted signal strength measurements for two or more different communication networks.